This invention relates generally to stencil and screen prints, and particularly to improving the quality of such stencil and screen prints.
Stencils and screens to apply patterns or printable material on substrates have been used in many contexts. For example, such a patterned adhesive layer, as described in Farnworth et al., U.S. Pat. No. 5,286,697, which is hereby incorporated by reference, may be used to attach a semiconductor die to a lead frame. Farnworth et al. describes the use of a patterned screen to deposit an adhesive layer on the die so that the die can be mounted to the lead frame. The screen is patterned so that the streets between individual dies on a wafer are free from adhesive to permit later sawing of the wafer into separate the dies, and so that the bond pads are also free fiom adhesive to permit later connection of the bond wires between the lead fingers of the lead frame and the bond pads.
Farnworth et al. specifically teaches a screen printing process in which the patterned screen is situated between the wafer and a liquid adhesive nozzle. Once the screen is properly aligned over the wafer, liquid adhesive is released from the nozzle, coating the wafer with adhesive in the desired pattern. The screen is then removed from the wafer, and the dies of the wafer are attached to a lead frame and separated from one another. The application of liquid adhesive on a wafer using a screen as described in Farnworth et al. is similar to a silk screen process used for printing artwork, such as on T-shirts. This same stencil and screen printing process similar to the silk screen process used for printing artwork on T-shirts has also been utilized in other semiconductor applications. Such semiconductor applications include printing a patterned nonconductive polyimide barrier for flip chips, printing conductive adhesive bumps or solder bumps, and printing conductive ink on flexible printed circuit boards (PCBs).
However, it has been found that a shortcoming to such screen and stencil printings is the migration or overflow of the printable material (such as the adhesive or the ink) underneath the bottom of the screen, which causes the substrate to which the printable material is being applied (such as the wafer or the printed circuit board) to stick to the screen. This results in a poorly patterned layer. More significantly, migration underneath the bottom of the screen shortens screen life, which is defined as the number of consecutive uses, or prints, of the screen without cleaning of the screen. Migration underneath the bottom of the screen forces cleaning of the screen between prints to ensure consecutive clear prints. Reduced screen life therefore slows down the screen and stencil printing process, and adds cost to the process.
The above-mentioned shortcomings are addressed by the present invention, which will be understood by reading and studying the following specification. The invention describes a stencil and screen used to assist in the application of a patterned printable material layer on a substrate. For example, the stencil or screen may be used to apply an adhesive to a semiconductor die, which is typically a part of a semiconductor wafer. Preferably, the stencil or screen includes a coating applied to at least one surface of a patern to retard migration or overflow of the printable material onto the surface. Alternatively, the stencil or screen includes a second coating applied to at least one other surface of the pattern to promote spreading of the printable material onto the substrate to which it is being applied.
For example, the pattern may include a number of apertures. The side walls of these apertures are side surfaces of the pattern. During the printable material application process, the stencil or screen is aligned over the substrate to which the printable material is to be applied. The bottom surface of the screen or stencil is coated with a material such as polytetrafloroethylene or another polymeric material to retard running of the printable material onto the bottom surface (i.e., between the stencil or screen and the substrate). The top surface and side surfaces of the pattern are alternatively coated with a material such as tungsten to promote spreading of the printable material from the top and side surfaces to the parts of the object surface exposed underneath.
The material coating the bottom surface of the pattern, which is desirably a polymeric material, in accordance with an embodiment of the invention, has three advantageous aspects. First, it acts as a low surface tension layer to minimize printable material migration in order to provide better print definition. Second, it acts as a gasket layer to constrain the flow of the printable material. Third, it acts as a buffer layer to minimize the stencil height inconsistency caused by defects, such as burr, on the bottom of the stencil and the top surface of the substrate onto which the printable material is applied.
A stencil or screen according to an embodiment of the invention therefore increases life of the stencil or screen. That is, consecutive prints can be made using a stencil or screen according to an embodiment of the invention without cleaning. This results in faster screen printing and reduces cost of the printing. Still other and further advantages, aspects and embodiments of the invention will become apparent by reference to the drawings and by reading the following detailed description.